Every beginning photographers red headed stepchild. When first starting out, this may be the hardest part of the photography trinity to understand. People use terms like "f-stops" and "DOP" but just like at the doctors office, sometimes we need people to explain things in terms we understand or at least define the terms they're using. So here we go, grab a cup a joe and a cupcake, this might be a long one.
DOF/Depth of Field: The easiest way to explain this is when you focus on an object or point and you hear the beep from the camera, the DOF is what is in focus in front and behind that focal point. The focal point will be the sharpest point across the image and will get decreasingly sharp the further forward or back you go in the image.
F-Stop/Stops: You've heard this term, but it usually stops you in your tracks... that was bad. On your lens, you will have a number after the focal length of the lens i.e. 50mm f/1.8. What this is telling you is that the lowest f-stop available to use is 1.8 and goes up from there. We will get into what these stops do.
Focal Plane: The focal plane an imaginary wall that starts at your focal point. Let's say you focus on your subjects eyes. The focal plane would be a wall facing you in line with the subjects eyes. If you set your camera down and walked forward your subject, you would run right into that imaginary wall of sharpness. That focal plane will be the sharpest in the picture and will get decreasingly sharp the further forward or back you go in the image(you've heard this before, it's because DOF and Focal Plane go hand in hand)
Bokeh: The beautiful out of focus area in our pictures. Bokeh actually has a quality, some lenses give you creamier bokeh and some give more rigid bokeh. A lot has to do with the blades on the shutter and your sensor, but that's for another day in another battle.
Ok, now that we have some definitions, let me explain in the best way that I can, what you really want to know about aperture and how it functions. In portrait sessions, we all love that creamy bokeh in the foreground and background. It blurs out all the unnecessary details and helps the viewer focus on the subject. What we call this is a very SHALLOW DEPTH OF FIELD(DOF). But how do we achieve this??
Landscape photography, sharp vivid images usually. Almost everything is in focus, from the small cracks in the boulders to the green grass right in front of you. Everything looks crisp and clear, so much so you feel you know every detail of the scene. I bet you can guess what we call this, a very DEEP DOF. But how do we achieve this?
The best way I can explain this is that the lower the number f-stop, the shallower the DOF or the thinner that imaginary wall is(FOCAL PLANE). So, for my 50mm f/1.8, if my f-stop is at 1.8(shooting wide open) the shallowest DOF will be achieved by using f/1.8 and the more bountiful bokeh I will have in front of and behind the point of focus(the beep). Now, if I want an incredibly sharp and deep DOF, I will need to use a higher f-stop, the highest being f/22 or f/infinity. Again, we would use f/22 if we were shooting landscapes or things we want all the details seen.
Ok, so how do changing these f-stops change your picture, other than the DOF/what's sharp and what's not? Aperture is all about light and the amount of light being let in. I could confuse you and tell you about how it works and even draw you a little diagram showing why it does what it does, but I feel like that may confuse you. For now, don't let numbers confuse you or represent amounts of light etc.
The lower the f-stop, the more light the camera allows to hit the sensor. So if I'm shooting "wide open" at f/1.8(some lenses go to f/1.2) I'm allowing the maximum amount of light in. Because I'm doing that, the other two parts of the trinity need to change, for instance, I need to increase the shutter speed. If I'm at a high f-stop, like f/22, I'm letting in a very little amount of light, so I need to slow down the shutter speed. Try this on your camera if you have it around. Shoot a picture of a vase or something on your coffee table where there's a decent amount of light. Just put your ISO at 800(the next blog) and put your aperture at the lowest stop you have and your shutter speed at 1/60. It's probably blown out with a lot of light coming in. Now increase your shutter speed to maybe 1/100 or 1/200. You should be getting closer to a properly exposed picture with a shallow DOF.
Alright, that's a lot of info and I'm only going to do one more practical application section. You could probably just call this the study guide. If you have a portrait session, you're closer to your subject and you just want the eyes of the subject in focus, what would be the f-stop you would use on a 50mm f/1.8 lens? Answer, f/1.8. Now lets say you want the nose and eyes in focus as well, what would you do?? Maybe shoot at f/2.8 or f/4. Let's say you have a few people in the picture now and they're stacked behind each other like threes company, what would you have to do to make sure everyone is in focus? Probably shoot at a higher f-stop, maybe f/8. Now lets say you have a big family and you want to make sure everyone is sharp and in focus... f/11? Alright... last but not least, your family is outdoors standing in front of the Niagara Falls and you want to be sure to get that in focus as well... f/22. Make sense??? Remember, the trinity is like a stick shift/manual transmission... when you brake, switch gears, change one thing, the others need to make changes as well. When you change the f/stop, you need to make the correct changes to the others. Rule of thumb is, for every stop you change on one, you compensate a stop in the other... for instance, going from f/2.8 to f/4, you would adjust the shutter from 1/160 to 1/100 and vice versa.
I realize that's a bunch of information and I could go on and on about aperture, but I'll stop for now. If you have any questions, please let me know/contact me.